With the advent of the MST (mesosphere-stratosphere-troposphere) radar technique the atmosphere from 0 to 100 km can be observed with a single radar system. Several different mechanisms have been identified as causing the echoes observed by MST radars. Two primary mechanisms are scattering from turbulent irregularities and Fresnel (or partial) reflection from horizontal strata. Two additional mechanisms that apply primarily to the upper mesosphere are meteor scatter and thermal (or incoherent or Thomson) scatter. Below the stratopause the refractive index structure responsible for the echoes arises solely from neutral atmosphere fluctuations; above the stratopause the radar cross section of such fluctuations is enhanced by the presence of free electrons (moreover, in the absence of small-scale neutral fluctuations the electrons alone can provide the scattered signal). This paper is an attempt to unify the scattering and reflection theories in the light of observational studies.

On the scattering and reflection mechanisms contributing to clear air radar echoes from the troposphere, stratosphere, and mesophere

Simultaneous and colocated comparisons of winds and tides by MF and meteor VHF radars have been made in the 85–94 km height range from July 1994 to June 1996 over London, Ontario, Canada. Results have been obtained for every month of the year. From these comparisons it is concluded that, in general, the MF spaced antenna technique and the meteor method both provide reliable means for synoptic studies of neutral air motions in the height range 85–94 km, at timescales of greater than 12 hours, and therefore are valuable tools in middle atmospheric research. However, we do find that some parameters seem to be estimated with greater precision than others. In particular, the measurements of the zonal long-term wind variations, and the semidiurnal tide in both the zonal and meridional directions, seem particularly robust and reliable, while there is somewhat less consistency between measurements of the zonal diurnal tide and the meridional monthly mean winds. The former problem is very likely to be an artifact of the strong diurnal variation in meteor count rates. However, we cannot claim that our meridional monthly mean wind agreement is always good, and these discrepancies deserve further investigation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96RS03467

Simultaneous and colocated observation of winds and tides by MF and meteor radars over London, Canada (43°N, 81°W), during 1994–1996

A study of inertial scale gravity wave motions in the region of the atmosphere between 30 and 60 km has been undertaken, using wind and temperature data derived from rocket-borne falling sphere density experiments performed over Woomera, Australia between 1962 and 1976. The gross features of the wave field compare favorably with those found in similar northern hemispheric studies. Wave propagation is found to be both vertically and horizontally anisotropic. A rotary spectral analysis indicates predominately upgoing wave energy, suggesting that the majority of sources of these waves lie below 30 km. A detailed statistical investigation of the waves, made using the Stokes parameters technique, reveals that phase progression is also highly directional in the horizontal, with a significant zonal component in summer, but with a strong meridional component in winter. Propagation towards the southeast is inferred in summer, with the waves possibly emanating from tropospheric sources in equatorial regions to the north of Australia. The technique also shows that, on average, the waves appear to have mean ellipse eccentricities ( = f/ω) around 0.4–0.45. Indirect estimates of a number of important wave parameters are made. In particular, \( \overline {u'w'} \) and \( \overline {v'w'} \) flux estimates are made over several height intervals. The vertical gradient of density weighted flux implies wave-induced mean flow accelerations of the order 0.1–1 ms-1day-1. This suggests that dissipating gravity waves are a significant source of the momentum residuals that are encountered in studies of satellite data from this region.

Falling Sphere Observations of Anisotropic Gravity Wave Motions in the Upper Stratosphere over Australia

Observations of the troposphere with a VHF radar using a large antenna for transmission and small spaced antennas for reception show that atmospheric velocities measured by the drifts method agree well with balloon winds. Many of the echoes from the middle troposphere appear to come from horizontally stratified layers. Studies of the irregularity structure by measurement of the spatial coherence of these echoes and of their angular spectra by interferometric techniques show that many echoes are caused essentially by specular reflection.

VHF radar studies of tropospheric velocities and irregularities using spaced antenna techniques

A model has been developed for simulating the effects of backscatter from scatterers advected with a mean background wind. This model has been designed to be as simple yet as realistic as possible, allowing the simulation of both spaced antenna and Doppler radars, and includes features such as aspect sensitivity, gravity wave perturbations, and turbulent motions. The model simulates the characteristics of radar backscatter very well at both MF and VHF. Results of the application of the full correlation analysis to model data generated for the spaced antenna configuration are presented, revealing good agreement with the model input velocity. The effects of the sampling time upon the full correlation analysis are investigated, suggesting an upper limit for the successful application of the technique. The triangle size effect of the full correlation analysis is also investigated, confirming that the major cause is the failure to properly compensate for the effects of noise. The application of techniques for the estimation of turbulent velocities and aspect sensitivity from model-generated data have also proven successful.

K.H. Lloyd

Papers

A comparison of partial reflection drifts with winds determined by rocket techniques—1